The invention relates to an exhaust gas catalyst composition, in the following “catalyst composition”, and a process for its preparation.
The reduction of nitrogen oxide emissions represents one of the greatest challenges in environmental protection. Several approaches have been followed to reduce NOx emissions for both mobile and stationary applications including combustion modification methods and denitrification of flue gases. The former, although NOx removal efficiency varies with the technology applied, cannot achieve more than 50-60% of removal efficiency. After-treatment of flue gases can achieve substantially larger efficiencies especially when a catalytic method is employed. Several type of catalysts have been tested which are active under different environments and conditions. The use of a large number of catalysts to eliminate NO is associated with different reaction pathways that can be divided as follows (1):    1. The selective catalytic reduction of NO with ammonia (herein after referred to as SCR), for stationary applications like power stations and chemical industrial plants.    2. The catalytic reduction of NO in the presence of CO, typical of automotive pollution control.    3. The catalytic reduction of NO in the presence of hydrocarbons, a method not in use commercially but potentially interesting for automotive and industrial pollution control.    4. The direct elimination of NO through decomposition for which a durable and stable catalysts has not yet been developed.    5. The sorbing of NO or NOx-trap catalysts.
Among these methods the most widely employed technology for stationary applications is SCR (2-4). It was introduced in the late 1970s for the control of NOx emissions in stack gases for thermal power plants and other industrial facilities. SCR plants are currently operating in USA, Japan, Europe and Far East for a total capacity of the order of 180000 MW. The SCR is based on the reduction of NOx with NH3 into water and nitrogen according to the reaction:4NO+4NH3+O2=4N2+.6H2O
The technology is operated commercially over metal-oxide SCR catalysts made of a homogeneous mixture of TiO2 (80-90 wt.-%), WO3 (6-10 wt.-%) and V2O5 (up to 3 wt.-%) which may contain some SiO2 (0-10 wt.-%) in the formulation. Titania is used as an active support of high surface area to support the active component V2O5 which is responsible for the activity of catalysts for NOx reduction at low and medium operation temperatures. It is also responsible for the oxidation of SO2 to SO3 when SO2 containing gases are delivered to the catalyst. Therefore, for high-sulfur content off-gases, its amount is kept low (below 1 wt.-%). WO3 (sometime also MoO3) is employed as a chemical/structural promoter to enlarge the temperature window of application. Silica is often used to improve the catalyst strength and stability. Commercial catalysts are employed as honeycomb monoliths due to several advantages over a packed bed arrangement: lower pressure drop, higher attrition resistance, less plugging by fly ash.
GB 1 495 396 describes a catalyst composition containing as active ingredients oxides from titanium, at least one of molybdenum, tungsten, iron, vanadium, nickel, cobalt, copper, chromium and uranium, and as optional component(s) tin and/or at least one of silver, beryllium, magnesium, zinc, boron, aluminium, yttrium, rare earth metal, silicon, niobium, antimony, bismuth, manganese, thorium and zirconium, which oxides are present as an intimate mixture.
EP 1 145 762 A1 describes a process for the preparation of a vanadia SCR-catalyst supported on titania. The process is characterized in that the catalyst is prepared by dispersing titania in an ammonium metavanadate solution, adjusting the pH of the solution to a value of 7.0-7.1, stirring the resulting suspension for a time for complete adsorption of the vanadium compound on titania, filtering the suspension and drying and calcining the resulting catalyst compound.
In spite of the fact that SCR technology is used worldwide there are still opportunities to improve catalytic performance especially in relation to the following issues: (i) to improve catalyst design in order to obtain at the same time a higher activity in NOx removal and a lower activity in SO2 oxidation; (ii) to limit ammonia slip and to improve the behaviour of the system under dynamic conditions; (iii) to extend the present applicable temperature range of SCR catalysts towards higher temperature up to 600° C. and to avoid deactivation which occurs at present catalysts when operated at high temperatures. It is in fact known that the activity of a V2O5/TiO2/SiO2 catalyst increases markedly with a rise in calcinations temperature up to 600-650° C. and then rapidly decreases. This is mainly due to phase transformation of TiO2 (anatase) into TiO2 (rutile) and consequent loss of BET surface area with changes in the chemical state of surface vanadium species. Solving these issues will pave the road for use of SCR also in mobile applications; the process using urea as reducing agent is in fact investigated intensively for use in diesel or gasoline lean-burn engines (5-6). The challenges for automotive applications are high SCR activity and improved thermal stability of vanadia-tungsta-titania catalysts in the temperature range 423-1 000 K. Such extreme operating temperatures (compared to “classic” SCR applications where temperature range of the order of 573-773 K are often encountered) are certainly of short duration and may occur at very high power output (low rpm and high load).